Method and apparatus for an enhanced RFID tag interrogator

ABSTRACT

An apparatus for an enhanced receiver for an RFID Tag Interrogator is described. The enhanced receiver has a zero crossing detector, a phase shifter, a pulse generator and a chopper. The zero crossing detector produces a sampling signal from the current transformer. The phase shifter modifies the sampling signal by producing a 90 degree phase shifted sampling signal. The pulse generator increases time duration of each pulse of the phase shifted sampling signal, which is then fed to the chopper. Thus, the chopper samples the backscattered carrier signal every 90 degree phase shift from the time when the backscattered carrier signal passes the zero crossing. This makes isolation of the backscattered signal from the carrier signal more effective.

FIELD OF THE INVENTION

The present invention generally relates to Interrogators for RFID Tagsutilizing backscatter modulation. In particular, the invention relatesto a method and apparatus for an enhanced Interrogator for RFID Tagsutilizing backscatter modulation.

BACKGROUND OF THE INVENTION

RFID systems today are well known and used for tracking, identifying andmonitoring a variety of items from shipping containers to library books.A typical RFID system would comprise several RFID Tags and at least oneInterrogator. The Interrogator communicates with the RFID Tags bytransmitting carrier signals modulated with data to the RFID Tags. TheRFID Tags in turn modulates the modulated carrier signals from theInterrogator using modulated backscattering and reflects and replies tothe Interrogator.

Commands and data modulated in the carrier signals are sent to the RFIDTag through the antenna via RF coupling. While the RFID Tag's receptionof the Interrogator's commands and data may possesses good signal tonoise ratio, the Interrogator's reception of the RFID Tag's reply maynot be so.

Amplitude of backscattered signals are generally low and depends on thedistance from the Interrogator's antenna to the RFID Tag. The amplitudedifference may be as disparate as 120 dB below the carrier signal'samplitude. This results in the amplitude of the RFID Tag's reply beingin the range of 100 uV on a carrier signal of 60 volts peak to peak.Furthermore, the modulation frequency of backscattered signals are lowcompared with the frequency of the carrier signal. The modulationfrequency of a backscattered signal is typically a few hundred kilohertzfor a carrier signal having a frequency of about 13.56 megahertz. Thesedisproportionate differences in amplitudes and frequencies betweencarrier and backscattered signals have caused problems with dataretrieval.

Presently, research and developmental efforts spent on isolating thebackscattered signals from carrier signals have yielded results whichare far from satisfactory. The attenuation of the large carrier signalsinherently reduces the backscattered signals, giving rise to poor noiseperformance. In attempting to raise the level of the backscatteredsignals, some manufacturers have resorted to simply increasing thetransmission power. This method not only fails to improve the signal tonoise ratio of the Interrogator's reception of the RFID Tag's reply butmay even run foul of transmission power limitations.

Another concern is the existence of “blackholes” in the activation fieldof an Interrogator. These “blackholes” are regions in the activationfield where the replies of the RFID Tags cannot be correctly received bythe Interrogator. As the backscattered signals take the form of DoubleSide Band Without suppression of the carrier signal, the side bands caninterfere with the carrier signal by way of multi-path reflection andphase delays to cause erroneous effects on the received backscatteredsignal. This may result in data inversion and data fallout. Theseeffects are especially common in Interrogators employing amplitudedemodulation techniques such as diode detectors. It is also occurs insingle antenna systems as both transmitted and received signalsoriginate from one point. This creates a higher chance for signals tointerfere and with only one reference point, higher difficulty inisolating the backscattered signal.

Another influence in the quality of backscattered data retrieval is thenoise in the transmitting circuit. Inherent noise in the transmissionwill often mask the backscattering signals.

The present invention prevents the erroneous recovery of backscatteredsignals from RFID Tags and provides extremely high signal to noise ratiofor the reception of backscattered signals. The present invention isalso designed for low inherent noise.

Conventional Interrogators typically are able to communicate with RFIDTags on a single backscattered frequency. Similarly, typical RFID Tagsonly communicate with Interrogators via single frequency backscatteredmodulation. Furthermore, there have not been requirements forInterrogators capable of simultaneous communication with a plurality ofRFID Tags operating on a plurality of different frequencies. There alsohave not been requirements for Interrogators capable of communicatingwith individual RFID Tags operating on more than one backscatteredfrequency.

The present invention further addresses the future need forInterrogators capable of simultaneously communicating with a pluralityof RFID Tags working on a plurality of different frequencies as well asRFID Tags individually operating on more than one backscatteredfrequency.

SUMMARY OF THE INVENTION

The present invention seeks to provide a method and apparatus for anenhanced RFID Tag Interrogator,

Accordingly, in one aspect, the present invention provides, an apparatusfor an enhanced receiver for an RFID Tag Interrogator having a currenttransformer and an antenna, the enhanced receiver comprising: a zerocross detector for receiving an input current waveform from the currenttransformer and for producing a sampling signal based on the inputcurrent waveform; a phase shifter for phase shifting the sampling signalby about ninety degrees to produce a phase shifted sampling signal; apulse generator for increasing time duration of each pulse of the phaseshifted sampling signal to produce a control signal for controlling achopper; the chopper for receiving and sampling a backscattered carriersignal from the antenna; and a bandpass filter for filtering away acarrier signal of the backscattered carrier signal.

In another aspect, the present invention provides, a method forreceiving a modulated backscattered carrier signal, steps comprising:producing a sampling signal comprising of a plurality of instantaneouspulses in accordance with zero crossing of a current waveform of themodulated backscattered carrier signal; phase shifting the samplingsignal by about ninety degrees; increasing time duration of theinstantaneous pulses by a predetermined duration to produce a controlsignal; and controlling a chopper with the control signal to sample themodulated backscattered carrier signal.

In a further aspect, the present invention provides, an apparatus for anRFID Tag Interrogator having a current transformer and an antenna, theapparatus comprising: an enhanced receiver; and a Variable BandpassMulti-Band Demodulator.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be more fullydescribed, with reference to the drawings of which:

FIG. 1 illustrates a block diagram of an RFID Tag Interrogatorincorporating the apparatus in accordance with the present invention;and

FIG. 2 illustrates the apparatus in accordance with the presentinvention.

DESCRIPTION OF THE INVENTION

A method and apparatus for an enhanced RFID Tag Interrogator in apreferred embodiment is described. It shall be apparent to one skilledin the art, however that the invention may be practiced without suchdetails. Some of the details may not be described at length so as not toobscure the invention.

Referring to FIG. 1 and FIG. 2, the apparatus in accordance with thepresent invention is illustrated. An Interrogator 10 having an enhancedreceiver 20 and a Variable Bandpass Multi-Band (VBMB) Demodulator 40 inaccordance with the present invention is illustrated. The enhancedreceiver 20 is coupled both to the current transformer 14 and theantenna 12 of the Interrogator 10. The enhanced receiver 20 is furthercoupled to the VBMB Demodulator 40.

The Interrogator 10 further comprises known portions such astransmitters and filters coupled to the antenna 12, the currenttransformer 14, and the enhanced receiver 20.

The enhanced receiver 20 comprises a zero cross detector 22, a phaseshifter module 24, a pulse generator 26, a chopper 28 and a bandpassfilter 30. The zero cross detector 22 is coupled both to the currenttransformer 14 of the Interrogator 10 and to the phase shifter 24. Thezero cross detector 22 receives an input of a current waveform of thebackscattered carrier signal received by the antenna from the currenttransformer 14. The zero cross detector 22 determines the time instantsat which the current waveform crosses over from positive to negative ornegative to positive and outputs sampling signal comprising of a seriesof instantaneous pulses. Each instantaneous pulse of the sampling signalindicates a time instant at which the current waveform of the receivedsignal has crossed over or under the zero value.

The zero cross detector 22 is further coupled to the phase shifter 24which allows the sampling signal to be phase shifted by about 90°. Thephase shifted sampling signal is then sent to a pulse generator whichincreases the sampling time of each of the series of instantaneouspulses of the phase shifted sampling signal to produce a control signal.The width of these pulses are determined by the sensitivity desired whensampling the backscattered carrier signal. The control signal which isthe output from the pulse generator 26 is then used to control thechopper 28 for sampling the backscattered carrier signal from theantenna 12.

It can thus be seen that the sampling of the backscattered carriersignal by the chopper 28 occurs at about every 90° phase shift from thetime when the backscattered carrier signal crosses over or under thezero value. This occurrence is due to the control signal from the pulsegenerator 26 being based similarly on the current waveform of thebackscattered carrier signal crossing over or under the zero value andbeing phase shifted by about 90° by the phase shifter 24.

The overall effect results in the sampling of the backscattered carriersignal at the location and time when the backscattered carrier signal isat its highest or lowest amplitude and detection of any amplitudechanges is easily isolated.

The output from the chopper 28 which is the chopped backscatteredcarrier signal is then sent to the bandpass filter 30 for filtering awayof the carrier signal. The bandpass filter 30 may be based on ceramic,surface wave or other common filter architectures. The bandpass filter30 also serves to perform image rejection. The output from the bandpassfilter 30 thus contains only the backscattered signal and a D.C voltagewithout the carrier signal. As this method is sensitive to both phasemodulation and amplitude, modulation, it forms the primary detector.

The backscattered signal from the bandpass filter 30 is then amplifiedand mixed with multiple frequencies and separately demodulated in theVariable Bandpass Multi-Band (VBMB) demodulator 40. The VBMB Demodulator40 comprises of a plurality of mixers 32 coupled to a plurality ofVariable Frequency Oscillators (VFOs) 34. The plurality of mixers 32 arefarther coupled to a plurality of backscattered signal demodulators 36.

This VBMB demodulator 40 can be heterodyne, super-heterodyne or directconversion demodulators. These VBMB demodulators may employ quadraturedetectors and receive signal strength indicators to recover bothFrequency Shift Keying and Amplitude Shift Keying based data. Phasedetectors and Phase Lock Loops can also be added to provide Phase ShiftKeying demodulation.

Image rejection and single side band operation enhances the rejection ofunwanted signals and noise while improving backscattered signal to noiseratio. Together with a transmitter with low noise floor, can eliminate“blackholes” in the interrogating field. The low noise transmitter andhigh backscatter sensitivity lowers the transmission power requirementand eases regulatory compliance.

The VBMB Deodulator 40 has the ability to simultaneously demodulatemultiple backscattered frequencies enabling “multi-band” operation. Thisis crucial for implementing systems with security features such asElectronic Article Surveillance (EAS).

An example of which is a RFID Tag replying via backscattered modulationto the Interrogator 10 at a frequency of 423 KHz after receiving acommand signal from the Interrogator 10. This is a command and replytype system whereby Interrogator 10 has communication priority. The samesystem may also consist of the RFID Tags operating a secondbackscattered frequency of 2.4 MHz once they are in the activation fieldof the Interrogator 10. The operation of this second backscatteredfrequency occurs independently of the Interrogator's command signal andis based on RFID Tags having the communication priority. Thisindependent operation of the RFID Tag is often for the implementing ofthe abovementioned EAS. The 2.4 MHz system co-exists with the 423 KHzsystem. With conventional single band systems, either the backscattereddata or the EAS signal is received, therefore losing the other signal.The present invention however advantageously allows both signals to bereceived simultaneously.

It will be appreciated that various modifications and improvements canbe made by a person skilled in the art without departing from the scopeof the present invention.

1. An enhanced receiver for an RFID Tag Interrogator having a currenttransformer and an antenna, said enhanced receiver comprising: a zerocross detector for receiving an input current waveform from said currenttransformer and for producing a sampling signal based on said inputcurrent waveform; a phase shifter for phase shifting said samplingsignal by about ninety degrees to produce a phase shifted samplingsignal; a pulse generator for increasing time duration of each pulse ofsaid phase shifted sampling signal to produce a control signal forcontrolling a chopper; said chopper for receiving and sampling abackscattered carrier signal from said antenna; and a bandpass filterfor filtering away a carrier signal of said backscattered carriersignal.
 2. A method for receiving a modulated backscattered carriersignal, steps comprising: a. producing a sampling signal comprising of aplurality of instantaneous pulses in accordance with zero crossing of acurrent waveform of said modulated backscattered carrier signal; b.phase shifting said sampling signal by about ninety degrees; c.increasing time duration of said instantaneous pulses by a predeterminedduration to produce a control signal; and d. controlling a chopper withsaid control signal to sample said modulated backscattered carriersignal.
 3. An apparatus for an RFID Tag Interrogator having a currenttransformer and an antenna, said apparatus comprising: an enhancedreceiver; and a Variable Bandpass Multi-Band Demodulators; wherein saidenhanced receiver comprises; a zero cross detector for receiving aninput current waveform from said current transformer and for producing asampling signal based on said input current waveform; a phase shifterfor phase shifting said sampling signal by about ninety degrees toproduce a phase shifted sampling signal; a pulse generator forincreasing time duration of each pulse of said phase shifted samplingsignal to produce a control signal for controlling a chopper; saidchopper for receiving and sampling a backscattered carrier signal fromsaid antenna; and a bandpass filter for filtering away a carrier signalof said backscattered carrier signal.
 4. The apparatus in accordancewith claim 3, wherein said Variable Bandpass Multi-Band Demodulatorcomprises: a plurality of variable frequency oscillators; a plurality ofmixers; and a plurality of demodulators; wherein said plurality ofmixers for receiving a plurality of backscattered signals from saidenhanced receiver and for mixing said plurality of backscattered signalswith variable frequencies from said plurality of variable frequencyoscillators; further wherein said plurality of demodulators forreceiving input from said plurality of mixers and for separatelydemodulating each of said plurality of backscattered signals.